I encountered a similar problem today. Wanted a minimal set of fields to process frequently so created a smaller struct, with only a subset of key/value pairs of the full one. Then I fetch data from the DB in a manner very similar to the one described here:
IOW I am getting a Map (well, list of maps in that particular example there) with only the key/value pairs I am interested in and pass it to struct!2
struct!(%MyMinimalStruct{}, my_map_from_db_query)
Seems almost suspiciously trivial… any gotchas?
Alternative could be to create another “model” with limited Ecto.Schema defnition. Might check that route too
You can do from x in query, select: %MyMinimalStruct{id: x.id, name: x.name}. A bit more verbose than the map/2 syntax, but no later mapping needed. If MyMinimalStruct is also a schema module there’s also Repo.load.
I think this approach is the right fit for me. It preserves Ecto’s type casting without relying on validations that feel out of place when loading from the DB. A changeset makes sense in the app-to-DB direction, but going the other way it’s redundant since the data has already passed validation (at least if Ecto is the only DB writer — otherwise falling back to a proper changeset is fine).
